1. Field of the Invention
This invention relates to well cement compositions having improved properties and methods, and more particularly, to cement compositions for constructing and repairing wells having improved properties such as acid solubility resistance at high temperatures and methods of using such compositions.
2. Description of the Prior Art
In carrying out completion and remedial operations in oil, gas and water wells, hydraulic cement compositions are commonly utilized. For example, hydraulic cement compositions are used in primary cementing operations during the construction of a well. Primary cementing involves the placement of a water slurry of a hydraulic cement into the annular space between the walls of the well bore and the exterior of a pipe such as casing disposed therein. The cement is permitted to set in the annular space thereby forming an annular sheath of hardened impermeable cement therein. The objective of the cement sheath is to physically support and position the pipe in the well bore and bond the pipe to the walls of the well bore whereby undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
In the operation of wells after primary cementing and other completion operations have been accomplished and produced fluids have been recovered from one or more subterranean formations penetrated by the well bore, problems relating to the undesirable flow of fluids with or without fine solids into or from undesirable locations in subterranean formations or the well bore often occur. The undesirable passage of fluids and/or fine solids can severely disrupt or terminate the operation of a well. Ordinarily, the undesirable passage of fluids involves the movement of oil, gas or water through small holes or cracks in the well casing; holes, cracks, voids or channels in the annular cement sheath between the casing and the walls of the well bore; very small spaces, called microannuli, between the annular cement sheath and the exterior surface of the casing or the walls of the formation; and permeable zones or fractures in gravel packs and/or subterranean formations. The problems of undesirable fluids passing through such openings are not unique, and the solutions have traditionally involved the placement of hydraulic cement compositions in the openings followed by permitting the cement compositions to harden and form impermeable plugs therein. These remedial techniques are generally referred to in the art as squeeze cementing techniques.
The success of squeeze cementing techniques whereby a water slurry containing hydraulic cement particles is squeezed into small openings is often a function of the size of the openings relative to the particle size of the cement as well as the overall properties of the slurry. If the particle size of the cement is greater than the openings, the cement cannot enter the openings and plugs therein will not result. Other properties of the cement slurry which affect the success or failure of primary and squeeze cementing techniques include fluid loss from the cement slurry and the time in which the slurry sets, i.e., the rate at which water is lost from the slurry as it contacts permeable zones in subterranean formations and the time after the cement slurry has been prepared before it thickens and sets during which the cement slurry must be placed in a desired location in a well.
When a well contains acid as a result of being stimulated with a fluid containing acid or other reason, or the fluids of the formations penetrated by the well bore are of a corrosive nature, the success or failure of cementing completion and remedial operations in the well can depend on the acid resistance of the hydraulic cement composition, both before and after the cement composition hardens into a solid mass.
Heretofore, methods of performing completion and remedial cementing operations in oil and gas wells have been developed wherein the hydraulic cement slurries used contain fine cement particles. For example, squeeze cementing methods which utilize hydraulic cement consisting of discreet particles having a particle size no greater than about 30 microns are disclosed in U.S. Pat. No. 5,121,795 issued Jun. 16, 1992, and primary cementing methods utilizing such hydraulic cement are disclosed in U.S. Pat. No. 5,125,455 issued on Jun. 30, 1992. The fine particle size hydraulic cement compositions described in the above patents can readily enter very small openings, can be formulated utilizing low ratios of water to cement, produce set cements having high compressive strengths, have low heats of hydration, expand upon setting and have other properties which are highly beneficial in conducting well cementing operations.
A variety of hydraulic cement slurry set retarding additives have been developed which can be utilized in hydraulic cement slurries of both normal cement particle size and ultra fine cement particle size to provide sufficient time between slurry formation and placement to conduct well cementing operations. Various hydraulic cement slurry fluid loss control additives have also heretofore been developed and used which are well known to those skilled in the art. However, in well cementing applications wherein the hydraulic cement is subjected to acid attack at relatively high temperatures, satisfactory acid resistant cement compositions and methods have not been available. Thus, there is a need for improved well completion and remedial cementing compositions and methods wherein the compositions have good acid solubility resistance as well as other required properties.
Also, a variety of dispersing agents have been developed and used heretofore which facilitate the preparation of well cement compositions, i.e., improve the mixability of the particulate solid cement with water and other cement composition components. While such dispersing agents have achieved varying degrees of success, there is still a need for improved cement composition mixability, especially in the preparation of cement compositions having low water to cement ratios.